Direct sequence spread spectrum used in CDMA systems generates a pseudorandom code and multiplicatively modulates a narrowband, data-modulated signal with the pseudorandom code to spread it over the radio frequency (RF) spectrum. Other signals spread with different, preferably orthogonal, codes may overlap in frequency and time and yet may be distinguished at a receiver designed to demodulate or despread a particular signal using its particular pseudorandom code. The frequency spectrum occupied by the spread spectrum signal is contiguous meaning that the transmitted signal occupies a particular frequency band of the spectrum continuously across that frequency band; there are no portions of the spectrum in that frequency band that are not substantially occupied by the spread signal. That continuous occupation across the spectrum is an unavoidable property of a pseudorandom code because the spectrum is the Fourier transform of the pseudorandom code's autocorrelation function. By definition, pseudorandom code has an Dirac or unit impulse response function autocorrelation, so its Fourier transform is flat and continuous in the frequency domain.
Frequency hopping (FH) spread spectrum uses non-contiguous spectrum with all of the signal energy appearing instantaneously in one FH channel. But that channel moves rapidly from instant-to-instant or hop-to-hop. The time during which the energy appears in any one FH channel is called the “dwell time”. The interference characteristics of frequency hopping systems to themselves and to or from other systems is therefore quite different than Direct Sequence (DS) CDMA systems. Frequency hopping transmissions tend to create, and be tolerant of, high levels of momentary, narrowband interference with a low probability of incidence. Direct sequence transmissions create, and are tolerant of, low levels of continuous, wideband interference. In scenarios where there are large distance variations between wanted and unwanted transmitters and receivers, called the “high near/far ratio” scenario, frequency hopping generally provides superior performance. In a scenario where wanted and unwanted radios are at equal distance from each other, such as satellite systems that have dedicated spectrum not shared with other terrestrial services, direct sequence may provide better performance. Direct sequence spread spectrum (DSSS) is also used in many modem wireless communications systems, like cellular systems, where near/far ratios may be managed by commanding a mobile terminal to communicate via the nearest base station.
Until now, it has only been possible to use direct sequence spread spectrum with a contiguous spectrum. For example, direct sequence could not be used by licensees who, for historical or other reasons, found themselves with a fragmented or non-contiguous spectrum. Non-contiguous means that the transmitted signal simultaneously occupies multiple portions of the spectrum across a particular frequency band so that there are portions of the spectrum in that band that are not occupied by the spread signal.
Another problem concerns multi-path propagation. When signal echoes are received via delayed paths, and the echo path delay is more than a small fraction of the duration of a modulation symbol, multi-path distortion is produced in which successive symbols appear to be mixed. This multi-path distortion effect is also known as Intersymbol Interference (ISI). Signals suffering from ISI may be decoded using one of the various types of equalizers, such as the Viterbi Maximum Likelihood Sequence Estimation algorithm, a transversal equalizer, a decision feedback equalizer, or hybrids thereof. A RAKE receiver for direct sequence spread spectrum signals performs a similar function but in a different way.
Other solutions to multi-path distortion include splitting a data stream into multiple, slower speed data streams, the symbol duration of each now being much longer than the multi-path echo delay, and transmitting the streams in parallel over separate channels. The separate channels are contiguous, narrowband channels within an allocated bandwidth, and this type of transmission was variously known as “multi-tone modem modulation,” “Kineplex,” and when the channel spacing of the narrowband channels bears a particular relationship to the data rate on each channel, Orthogonal Frequency Division Multiplexing (OFDM) of the separate channels. OFDM is not spread spectrum because the total bandwidth occupied is commensurate with the total underlying data rate. Two OFDM signals should not overlap as they do not have a distinguishing spread-spectrum code that would allow a receiver to separately distinguish them.
In any event, it would be desirable to use Spread Spectrum Code Division Multiple Access systems in non-contiguous spectrum and to be able to reduce or minimize adverse multi-path effects on such non-contiguous spectrum communications.